Polymeric photoinitiators

ABSTRACT

A polyalkylether photoinitiator of the general formula I, R 1 (A 1 ) r -(R 2 (A 2 ) m -O) o —(R 3 (A 3 ) n -O) p —R 4 (A 4 ) s  I wherein R 1 , R 2 , R 3 , R 4  and m, n, o, p, r and s are as defined herein and A 1 , A 2 , A 3  and A 4  are identical or different photoinitiator moieties.

This is a national stage of PCT/DK11/050055 filed Feb. 23, 2011 andpublished in English, which has a priority of Denmark no. PA 2010 70065filed Feb. 23, 2010, and Denmark no. PA 2011 70007 filed Jan. 7, 2011,hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to novel polymeric photoinitiators as wellas to applications thereof and methods of producing polymericphotoinitiators, with polyalkylethers as an integral part of thepolymeric system. Photoinitiator moieties are present as pendant groupson a polymeric backbone.

BACKGROUND OF THE INVENTION

Curing of coatings through ultraviolet (UV) radiation requires efficientmethods of initiating the chemical reaction responsible for the curingprocess. Cross-linking of polymeric material through generation ofradical species upon irradiation with UV light is widely used to producehydrogels for medical device coatings. Also, the paint and lacquerindustry makes use of UV initiated curing of acrylates, wherephotoinitiators in many cases are employed. These two examplesillustrate the diversity of UV curable coatings, where up until now theUV active component in the coating recipe relies on molecules withcomparable low molecular weight. The UV active components are partiallyfree to diffuse to the surface of the cured material thereby renderingthese substances exposed to the environment. Higher molecular weightphotoinitiators, in particular polymeric photoinitiators, havecomparably higher intrinsic viscosities which most likely result inlonger diffusion times through a matrix. Migration of the UV activesubstances to the surface is therefore diminished when polymericphotoinitiators are used as opposed to lower molecular weightphotoinitiators. Scarce literature found in scientific and patentpublications within the topic of polymeric photoinitiators suggests thatdevelopment of such polymers could lead to novel applications andpresent solutions for existing needs, such as provide a material withnegligent migration of substances to the surface. Available literaturediscussed in the following, outlines previous work within the field ofpolymeric photoinitiators, with focus on work relevant forphotoinitiators with a polyalkylether backbone.

Some descriptions of polymeric photoinitiators are found in scientificliterature, where for example 4-amino-4′-[4-aminothiophenyl]benzophenoneis polymerized with toluene-2,4-diisocyanate (J. Wei, H. Wang, J. Yin J.Polym. Sci., Part A: Polym. Chem., 45 (2007), 576-587; J. Wei, H. Wang,X. Jiang, J. Yin, Macromolecules, 40 (2007), 2344-2351). Examples of theuse of this photoinitiator to polymerize acrylates are also given inthis work. A similar strategy is also discussed in J. Wei, F. LiuMacromolecules, 42 (2009), 5486-5491, where4-[(4-maleimido)thiophenyl]benzophenone was synthesized and polymerizedinto a macromolecular photoinitiator.

A variety of polymeric photoinitiators other than benzophenone basedstructures are discussed in T. Corrales, F. Catalina, C. Peinado, N. S.Allen J. Polym. Sci., Part A: Polym. Chem., 159 (2003), 103-114.Polymers with thioxanthone, benzil, anthraquionone, camphorquinone,benzoin ether, acylphosphine oxide and silane functionalites in themacromolecules are discussed in terms of efficiency in comparison to lowmolecular weight analogues. In some examples, the photoinitiationactivity of the polymeric photoinitiator was one order of magnitudehigher than the mixture of the corresponding low molecular weightanalogues. This increase in activity in the particular case was ascribedto efficient excitation energy transfer between different moietiespresent on the polymer chain. One other explanation could be theprevention of recombination of radicals formed on the photoinitiatorsites as the initiators are “tied” onto a polymer chain.

Some patent literature discusses polymeric photoinitiators. One exampleis found in US 2007/0078246 where different aromatic ketone systems aresubstituted on a siloxane polymeric chain. Rates of photopolymerizationreactions are then used to prove an enhanced performance of thepolymeric photoinitiators as opposed to low molecular weight analogues.

As a further example, benzophenone derivatives with pendant alkyl ethersubstitutents have been described in WO 96/33156, but the primaryproperties described were related to migration of the photoinitiators tothe surface of a coating. The benzophenones are not repeating units inthe polymer and the polymers described in WO 96/33156 can be consideredend-functionalized with benzophenone moieties.

A related type of photoinitiator class is described in WO 2009/060235,where thioxanthone moieties are attached to an oligomeric backbone. Itis particularly specified that the thioxanthone polymericphotoinitiators have molecular weights below 2000 g/mol.

Several photoinitiators (e.g. benzophenone, anthraquinone) with pendantpolyalkyl ethers are described in WO 97/49664. Common to thesephotoinitiators is that the polyethylene glycol moieties attached to thephotoinitiators have preferred molecular weights in the range of 150 to900 Da and furthermore only one photoinitiator moiety is present perpolymeric photoinitiator.

Related to the photoinitiators described in WO 96/33156, similarstructures are described in WO 98/51759 where benzophenone derivativeswith pendant alkyl ether groups are presented. The main focus of theinvention described in WO 98/51759 is oxygen inhibition and lowmigration properties of the photoinitiator.

WO 03/033492 discloses thioxanthone derivatives attached to apolyhydroxy residue. However, these polyhydroxy groups have only up to 6hydroxy groups present in the chain.

As a final example of polymeric photoinitiators described in the patentliterature, U.S. Pat. No. 4,602,097 details an invention related towater soluble photoinitiators where two photoinitiator moieties arebridged together by a polyalkylether of sufficient length to make itwater soluble.

Common to all of the inventions described above from the patentliterature, is that the active photoinitiator sites are present asend-groups on for example a polyethylene glycol which falls outside thepresent invention. The present invention details polymericphotoinitiators in which the photoinitiator moiety itself is an integralpart of the repeating unit in the polymeric photoinitiator.

OBJECT OF THE INVENTION

It is an object of embodiments of the present invention to provide novelpolymeric photoinitiators as well as to provide means and methods fortheir production. The invention also provides a method for curing amatrix composition and a cured matrix composition obtainable via saidmethod.

SUMMARY OF THE INVENTION

One aspect of the present invention is to provide access to polymericphotoinitiators with the general motif shown in FIG. 1, in particularsystems with a polyalkylether as the backbone polymer on which pendantphotoinitiators moieties are covalently attached.

So, in a broad aspect, the present invention relates to a polyalkyletherphotoinitiator of the general formula IR₁(A₁)_(r)-(R₂(A₂)_(m)-O)_(o)—(R₃(A₃)_(n)-O)_(p)—R₄(A₄)_(s)  Iwherein R₂ and R₃ are independently at each occurrence identical ordifferent, linear or branched alkylene or cycloalkylene groups; whereinR₂ and R₃ may be substituted with one or more substituents selected fromCN; azides; esters; ethers; amides; halogen atoms; sulfones; sulfonicderivatives; NH₂ or Nalk₂, where alk is any C₁-C₈ straight chain alkylgroup, C₃-C₈ branched or cyclic alkyl group;R₁ and R₄ are independently at each occurrence identical or different,linear or branched alkyl or cycloalkyl groups or aryl groups or areindependently at each occurrence selected from H, OH, CN, halogens,amines (e.g. —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25alkyl groups), amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″ arealkyl groups, suitably C1-C25 alkyl groups), alcohols, ethers,thioethers, sulfones and derivatives thereof, sulfonic acid andderivatives thereof, sulfoxides and derivatives thereof, carbonates,isocyanates, nitrates, acrylates, polyethylenes, polypropylenes,polyesters, polyamides, polyacrylates and polyurethanes; and when R₁ andR₄ are alkyl and aryl groups, they may be substituted with one or moresubstituents selected from CN; OH; azides; esters; ethers; amides;halogen atoms; sulfones; sulfonic derivatives; NH₂ or Nalk₂, where alkis any C₁-C₈ straight chain alkyl group, C₃-C₈ branched or cyclic alkylgroup;o and p are each a real number from 0-5000 provided that o+p>0;m and n are each a real number from 0-10,provided that m+n>0;r and s are each a real number from 0-5; andA₁, A₂, A₃ and A₄ are identical or different photoinitiator moieties.

The indices m, n, o, p, r and s in the general formula I represent anaverage/sum and the formula I thereby represents alternating, periodic,statistical/random, block and grafted copolymers. As an example of arandom copolymer may be mentioned the copolymer ABAAABABBABA having theformula A₇B₅.

An example of the identity of formula I applied to a photoinitiatordescribed in the present invention is given in Scheme 1.

R₂ and R₃ can be selected from any alkylene group having up to 25 carbonatoms and include both branched and straight chain alkylene groups.Exemplary, non-limiting alkylene groups include methylene, ethylene,propylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene,in the normal, secondary, iso and neo attachment isomers. Exemplary,non-limiting cycloalkylene groups include cyclopropylene, cyclobutylene,cyclopentylene and cyclohexylene.

As set out above, the alkylene groups R₂ and R₃ may be substituted with,apart from the photoinitiator moieties, substituents such as CN, azides,esters, ethers, amides, halogen atoms, sulfones, sulfonic derivatives,NH₂ or Nalk₂. “alk” is any C₁-C₈ straight chain alkyl group, C₃-C₈branched or cyclic alkyl group. Photoinitiator moieties can becovalently linked to R₂ and/or R₃ as designated by R₂(A₂) and R₃(A₃),where A₂ and A₃ can be any of the photoinitiator moieties describedherein.

R₁ and R₄ may independently be at each occurrence identical ordifferent, linear or branched alkyl or cycloalkyl groups. R₁ and R₄ canbe selected from any alkyl group having up to 25 carbon atoms andinclude both branched and straight chain alkyl groups. Exemplary,non-limiting alkyl groups include methyl, ethyl, propyl, butyl, pentyl,hexyl, heptyl, octyl, nonyl, in the normal, secondary, iso and neoattachment isomers. Exemplary, non-limiting cycloalkyl groups includecyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl.

R₁ and R₄ can also be selected from aryl groups, such as any aromatichydrocarbon with up to 20 carbon atoms. Exemplary, non-limiting arylgroups include phenyl, naphthyl, furanyl, thiophenyl, pyrrolyl,selenophenyl, and tellurophenyl. R₁ and R₄ can also be H, OH, CN,halogens, amines (e.g. —NR′R″, where R′ and R″ are alkyl groups,suitably C1-C25 alkyl groups), amides (e.g. —CONR′R″ or R′CONR″—, whereR′ and R″ are alkyl groups, suitably C1-C25 alkyl groups), alcohols,ethers, thioethers, sulfones and derivatives thereof, sulfonic acid andderivatives thereof, sulfoxides and derivatives thereof, carbonates,isocyanates, nitrates, acrylates. Furthermore, R₁ and R₄ can be selectedfrom polymeric entities such as polyacrylates, polyethylenes,polypropylenes, polyesters, polyamides and polyureathanes. The molecularweight of said polymeric entities is typically in the range of 50-50,000Da.

In some cases, when R₁ and R₄ are alkyl and aryl groups, they may besubstituted with, apart from the photoinitiator moieties, substituentssuch as CN, OH, azides, esters, ethers, amides (e.g. —CONR′R″ orR′CONR″—, where R′ and R″ are alkyl groups, suitably C1-C25 alkylgroups), halogen atoms, sulfones, sulfonic derivatives, NH₂ or Nalk₂,where alk is any C1-C8 straight chain alkyl group, C3-C8 branched orcyclic alkyl group. Photoinitiator moieties can be covalently linked toR₁ and/or R₄ as designated by R₁(A₁) and R₄(A₄), where A₁ and A₄ can beany of the photoinitiator moieties described above.

The indices m, n, o, p, r and s are defined as follows:

o and p are each a real number from 0-5000 provided that o+p>0;

m and n are each a real number from 0-10, provided that m+n>0;

r and s are each a real number from 0-5.

Suitably, o and p are each from 0-3000, preferably 0-2000. Suitably, oand p are each independently 1 or greater, e.g. 10 or greater. Theintegers m and n may be from 0-8, preferably 0-5, provided that m+n>0.Suitably, m+n≧1. In a particular aspect, m=1 and/or n=1. When m=1 andn=0, the ratio o:p may be at least 1:1000, preferably at least 1:500.

Typically, r and s are each from 0-4, preferably 0-2. Suitably, r and sare independently 1 or greater, e.g. 1 or 2.

The polyalkylether photoinitiator according to the invention may have amolecular weight between 5 and 10,000 kDa, preferably between 10 kDa and1,000 kDa, more preferably between 15 kDa and 500 kDa.

The photoinitiator moieties can be selected from2-hydroxy-2-methyl-propiophenone, benzophenone, thioxanthone, benzil,anthraquionone, camphorquinone, benzoin ether, acylphosphine oxide andsilane derivatives but the choice of photoinitiator moieties is notrestricted to these classes of compounds. As one example of moietiespotentially present in the polymeric photoinitiator is amines (e.g.—NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups).More details concerning the separate moieties are provided below underthe discussion in the detailed disclosure of the invention.

The present invention comprises aspects of coating formation employingthe polymeric photoinitiators through UV curing of either polymericphotoinitiator/matrix polymer composition and/or solely of the polymericphotoinitiator. This application extends among other things to paintsand lacquers, rubbers and protective coatings.

Ultimately, an aspect of the present invention is to use the polymericphotoinitiator solely in the coating composition eliminating the needfor mixing a photoinitiator and a matrix in the coating composition.

LEGENDS TO THE FIGURE

FIG. 1 illustrates a general motif of polymeric photoinitiators, withphotoinitiator moieties pendant on a polyalkylether.

FIG. 2 shows the ¹H-NMR spectrum ofpoly-co-ethyleneoxid-(4-(oxiran-2-ylmethoxy)phenyl)(phenyl)methanoneusing the polymerization procedure given for 1.

FIG. 3 shows the ¹H-NMR spectrum ofpoly-co-ethyleneoxid-(4-(oxiran-2-ylmethoxy)phenyl)(phenyl)methanoneusing the polymerization procedure given for 2.

FIG. 4 shows the curing profile for pristinepoly-co-ethyleneoxid-(4-(oxiran-2-ylmethoxy)phenyl)(phenyl)methanone (1)at 120° C.

FIG. 5 shows the curing profile for pristinepoly-co-ethyleneoxid-(4-(oxiran-2-ylmethoxy)phenyl)(phenyl)methanone (2)at 120° C.

DETAILED DISCLOSURE OF THE INVENTION

The present invention provides fabrication and applications of polymericphotoinitiators.

Photoinitiator and Photoinitiator Moieties

In the present invention, a photoinitiator is defined as a substance(other than a reactant) which, on absorption of light, generatesreactive species (ions or radicals) and initiates one or severalchemical reactions or transformation. One preferred property of thephotoinitiator is good overlap between the UV light source spectrum andthe photoinitiator absorption spectrum. Another desired property is aminor or no overlap between the photoinitiator absorption spectrum andthe intrinsic combined absorption spectrum of the other components inthe matrix. Good compatibility of the polymeric photoinitiator in thematrix consisting of material to be cured is also a property ofinterest.

In an embodiment of the polyalkylether photoinitiator according to theinvention A₁, A₂, A₃ and A₄ are identical or different photoinitiatormoieties selected from the group consisting of benzoin ethers, phenylhydroxyalkyl ketones, phenyl aminoalkyl ketones, benzophenones,thioxanthones, xanthones, acridones, anthraquinones, fluorenones,dibenzosuberones, benzils, benzil ketals, α-dialkoxy-acetophenones,α-hydroxy-alkyl-phenones, α-amino-alkyl-phenones, acyl-phosphine oxides,phenyl ketocoumarins, silane, maleimides, and derivatives thereof. Thephotoinitiator moieties A₁, A₂, A₃ and A₄ can also consist ofderivatives of the photoinitiator moieties listed.

In an embodiment of the polyalkylether photoinitiator according to theinvention A₁, A₂, A₃ and A₄ are identical or different photoinitiatormoieties selected from the group consisting of2-hydroxy-2-methyl-propiophenone, benzophenone, thioxanthone, benzil,anthraquionone, camphorquinone, benzoin ether, acylphosphine oxide,silane, and derivatives thereof. The photoinitiator moieties A₁, A₂, A₃and A₄ can also consist of derivatives of the photoinitiator moietieslisted.

In an embodiment of the polyalkylether photoinitiator according to theinvention A₁, A₂, A₃ and A₄ are identical photoinitiator moieties.However, A₁, A₂, A₃ and A₄ may be at least two different photoinitiatormoieties.

Suitably, at least one of A₁, A₂, A₃ and A₄ is a benzophenonephotoinitiator moiety. At least A₂ and A₃ may be benzophenonephotoinitiator moieties.

The photoinitiators of the invention preferably are efficient intransforming light from the UV or visible light source to reactiveradicals which can abstract hydrogen atoms and other labile atoms frompolymers and hence effect covalent cross-linking. Optionally, amines,thiols and other electron donors can be either covalently linked to thepolymeric photoinitiator or added separately or both. Radicalphotoinitiators can be classified as either cleavable (Norrish type Ireaction) or non-cleavable (of which the Norrish type II reaction is aspecial case, see e.g. A. Gilbert, J. Baggott: “Essentials of MolecularPhotochemistry”, Blackwell, London, 1991). Upon excitation, cleavablephotoinitiators moieties spontaneously break down into two radicals, atleast one of which is reactive enough to abstract a hydrogen atom frommost substrates. Benzoin ethers (including benzil dialkyl ketals),phenyl hydroxyalkyl ketones and phenyl aminoalkyl ketones are importantexamples of cleavable photoinitiators moieties. Addition of electrondonors is not required but may enhance the overall efficiency ofcleavable photoinitiators according to a mechanism similar to thatdescribed for the non-cleavable photoinitiators below.

Recently a new class of β-keto ester based photoinitiators has beenintroduced by M. L Gould, S, Narayan-Sarathy, T. E. Hammond, and R. B.Fechter from Ashland Specialty Chemical, USA (2005): “NovelSelf-Initiating UV-Curable Resins: Generation Three”, Proceedings fromRadTech Europe 05, Barcelona, Spain, Oct. 18-20, 2005, vol. 1, p.245-251, Vincentz. After base-catalyzed Michael addition of the ester topolyfunctional acrylates a network is formed with a number of quaternarycarbon atoms, each with two neighbouring carbonyl groups. Upon UV orvisible light excitation these photoinitiators predominantly cleave by aNorrish type I mechanism and cross-link further without any conventionalphotoinitiator present, and thick layers may be cured. Suchself-initiating systems are within the scope of the present invention.

Excited non-cleavable photoinitiators do not break down to radicals butabstract a hydrogen atom from an organic molecule or, more efficiently,abstract an electron from an electron donor (such as an amine or athiol). The electron transfer produces a radical anion on thephotoinitiator and a radical cation on the electron donor. This isfollowed by proton transfer from the radical cation to the radical anionto produce two uncharged radicals; of these the radical on the electrondonor is sufficiently reactive to abstract a hydrogen atom from mostsubstrates. Benzophenones and related ketones such as thioxanthones,xanthones, anthraquinones, fluorenones, dibenzosuberones, benzils, andphenyl ketocoumarins are important examples of non-cleavablephotoinitiators. Most amines with a C—H bond in α-position to thenitrogen atom and many thiols will work as electron donors.

Another self-initiating system based on maleimides has also beenidentified by C. K. Nguyen, W. Kuang, and C. A. Brady from AlbemarleCorporation and Brady Associates LLC, both USA (2003): “MaleimideReactive Oligomer”, Proceedings from RadTech Europe 03, Berlin, Germany,Nov. 3-5, 2003, vol. 1, p. 589-94, Vincentz. Maleimides initiate radicalpolymerization mainly by acting as non-cleavable photoinitiators and atthe same time spontaneously polymerize by radical addition across themaleimide double bond. In addition, the strong UV absorption of themaleimide disappears in the polymer, i.e. maleimide is a photobleachingphotoinitiator; this could make it possible to cure thick layers.

A blend of several photoinitiators may exhibit synergistic properties,as is e.g. described by J. P. Fouassier: “Excited-State Reactivity inRadical Polymerization Photoinitiator”, Ch. 1, pp. 1-61, in “Radiationcuring in Polymer Science and technology”, Vol. II (“Photo-initiatingSystem”), ed. by J. P. Fouassier and J. F. Rabek, Elsevier, London,1993. Briefly, efficient energy transfer or electron transfer takesplace from one photoinitiator to the other in the pairs[4,4′-bis(dimethylamino)benzophenone+benzophenone],[benzophenone+2,4,6-trimethyl-benzophenone],[thioxanthone+methylthiophenyl morpholinoalkyl ketone]. However, manyother beneficial combinations may be envisaged. So, in an embodiment ofthe invention, the photoinitiator moieties include at least twodifferent types of photoinitiator moieties. Preferably the absorbancepeaks of the different photoinitiators are at different wavelengths, sothe total amount of light absorbed by the system increases. Thedifferent photoinitiators may be all cleavable, all non-cleavable, or amixture of cleavable and non-cleavable.

In an embodiment of the polyalkylether photoinitiator according to theinvention at least one of A₁, A₂, A₃ and A₄ is a benzophenonephotoinitiator moiety.

Furthermore, it has recently been found that covalently linked2-hydroxy-1-(4-(2-hydroxyethoxy)phenyl)-2-methylpropan-1-one, which iscommercially available with the trade name Irgacure 2959, andbenzophenone in the molecule 4-(4-benzoylphenoxyethoxy)phenyl2-hydroxy-2-propyl ketone gives considerably higher initiationefficiency of radical polymerization than a simple mixture of the twoseparate compounds, see S. Kopeinig and R. Liska from Vienna Universityof Technology, Austria (2005): “Further Covalently BondedPhotoinitiators”, Proceedings from RadTech Europe 05, Barcelona, Spain,Oct. 18-20, 2005, vol. 2, p. 375-81, Vincentz. This shows that differentphotoinitiators may show significant synergistic effects when they arepresent in the same oligomer or polymer. Such covalently linkedphotoinitiators are also applicable within the present invention.

Each and every one of the above-discussed types of photoinitiators andphotoinitiator moieties may be utilised as photoinitiator moieties inthe polymeric photoinitiators of the present invention.

Polymeric Backbone (Photoinitiator Segment)

The polymeric backbone consists of a polyalkylether segment with thegeneral formula —(R₂(A₂)_(m)-O)_(o)—(R₃(A₃)_(n)-O)_(p)—, wherein R₂ andR₃ can be selected from any alkylene group having up to 25 carbon atomsand include both branched and straight chain alkylene and cycloalkylenegroups. Exemplary, non-limiting alkylene groups include methylene,ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene,nonylene, in the normal, secondary, iso and neo attachment isomers.Exemplary, non-limiting cycloalkylene groups include cyclopropylene,cyclobutylene, cyclopentylene and cyclohexylene.

In an embodiment of the polyalkylether photoinitiator according to theinvention, R₂ and R₃ are independently —CH₂CH₂— in which one or more Hatoms may be replaced by A₂ or A₃, respectively.

In an embodiment of the polyalkylether photoinitiator according to theinvention R₂ and R₃ are independently —CH(CH₃)CH₂— in which one or moreH atoms may be replaced by A₂ or A₃, respectively.

In some cases the alkylene groups may, apart from the photoinitiatormoieties, bear substituents such as CN, azides, esters, ethers, amides(e.g. —CONR′R″ or R′CONR″—, where R′ and R″ are alkyl groups, suitablyC1-C25 alkyl groups), halogen atoms, sulfones, sulfonic derivatives, NH₂or Nalk₂, where alk is any C1-C8 straight chain alkyl group, C3-C8branched or cyclic alkyl group. Photoinitiator moieties can becovalently linked to R₂ and/or R₃ as designated by R₂(A₂)_(m) andR₃(A₃)_(n), where A₂ and A₃ can be any of the photoinitiator moietiesdescribed above. The indices m, n, o and p are as set out above.

Curing

The ultraviolet spectrum is divided into A, B and C segments where UV Aextend from 400 nm down to 315 nm, UV B from 315 to 280 nm, and UV Cfrom 280 to 100 nm. By using a light source that generates light withwavelengths in the visible region (400 to 800 nm) some advantages areobtained with respect to the depth of the curing, provided that thephotoinitiator can successfully cure the material at these wavelengths.In particular, scattering phenomena are less pronounced at longerwavelength, thus giving a larger penetration depth in the material. Thusphotoinitiators which absorb, and can induce curing, at longerwavelength are of interest. By judicially choosing substituents on thephenone moieties, the absorption spectrum of the polymericphotoinitiator can to some extent be red-shifted, which would thenfacilitate curing at comparatively greater depths.

One class of compounds that absorbs light in the visible region istitanocenes. As an example bis-pentafluorophenyl titanocene is known toabsorb up to ˜530 nm (K. Meier, Coordination Chemistry Reviews, 111(1991), 97-110). This particular compound is also known to be stable upto 230° C. and is insensitive to acid treatment. A commerciallyavailable titanocene is available under the trade name Irgacure 784.

In the curing process, the photoinitiator cross-links or otherwisetransforms a surrounding polymer composition, hereafter referred to as amatrix, in a chemical process induced by light. The present inventiontherefore relates to the use of a polyalkylether photoinitiatoraccording to the invention for curing a matrix composition.

In addition, the invention provides a method for curing a matrixcomposition, said method comprising the steps of:

-   -   a. mixing the polyalkylether photoinitiator above with a matrix        composition;    -   b. curing the mixture obtained in step a. by exposing it to UV        radiation.

The present invention also relates to a matrix composition obtainablevia the method described herein.

The polymeric photoinitiators described here can facilitate curing of asurrounding matrix, but since the photoinitiators themselves arepolymers they can also “auto-cure”, meaning that the polymericphotoinitiators can solely constitute a coating composition that iscured upon UV irradiation. As such the pristine polymeric photoinitiatorcan be cured to form a cross-linked network, or the polymericphotoinitiator can be a constituent in a mixture which is subsequentlycured to form a cross-linked network.

Polymeric Photoinitiators of the Invention

Polyethylene Oxide Derived Photoinitiators.

The polymeric photoinitiators can be either synthesized by apolymerization reaction or grafted onto a polymeric backbone. A generalscheme for a direct synthesis of a polymeric photoinitiator with pendantphotoinitiator moieties based on epoxy-ring opening is shown in Scheme2, where the symbols from the general formula for the polymericphotoinitiators are exemplified.

The epoxide functionality used for the polymerization is obtainedthrough a reaction with epichlorhydrine, but might also be obtainedthrough a reaction with an allyl-derivative which is then subsequentlyoxidized with an oxidizing agent such as m-chloro-perbenzoic acid orhydrogen peroxide.

As illustrated in Scheme 2, attack of a nucleophile, either theinitiator or an alkoxide ion, occurs at the least substituted carbonatom on the epoxide present on the spacer group. Some reactionconditions, e.g. acidic conditions might favor the converse meaning thatthe most substituted carbon atom on the epoxide is attacked by thenucleophile. For simplicity, only polymerizations resulting in attack ofthe least substituted carbon atom in the photoinitiator attachedepoxide, is illustrated in the following, but the invention is not solimited.

With respect to substituents, R′₁, R′₂, R′₃ and R′₄ can be selected fromany alkyl groups having up to 25 carbon atoms and includes bothbranched, cyclic and straight chain alkyl groups. Exemplary alkyl groupsinclude methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl,nonyl, in the normal, secondary, iso and neo attachment isomers. R′₁,R′₂, R′₃ and R′₄ can also be selected from aryl groups, such as anyaromatic hydrocarbon with up to 20 carbon atoms. Exemplary aryl groupsinclude phenyl, naphthyl, furanyl, thiophenyl, pyrrolyl, selenophenyl,and tellurophenyl. In some cases the alkyl and aryl groups may bearsubstituents such as CN, azides, esters, ethers, amides (e.g. —CONR′R″or R′CONR″—, where R′ and R″ are alkyl groups, suitably C1-C25 alkylgroups), halogen atoms, sulfones, sulfonic derivatives, NH₂ or Nalk₂,where alk is any C1-C8 straight chain alkyl group, C3-C8 branched orcyclic alkyl group. R′₁, R′₂, R′₃ and R′₄ may also be H.

The spacer group, if any, may be selected from the same functionalgroups as R′₁, R′₂, R′₃ and R′₄ and additionally from groups mainlyconsisting of alkylethers, such as —(CH₂CH₂O)_(t)—, where t can be anyinteger from 0-100.

In an embodiment of the polyalkylether photoinitiator according to theinvention R₁=OH.

In an embodiment of the polyalkylether photoinitiator according to theinvention R₄=H.

As a first example, a polymerization of2-hydroxy-2-methyl-1-(4-(2-(oxiran-2-ylmethoxy)ethoxy)phenyl)propan-1-one(3) with either itself or ethylene oxide results in a (co)-polymer whichis a polymeric photoinitiator (Scheme 3). The precursor for thesynthesis of this polymer is2-hydroxy-1-(4-(2-hydroxyethoxy)phenyl)-2-methylpropan-1-one (Irgacure2959). A synthesis of (3) is outlined in U.S. Pat. No. 5,744,512.

An alternate route to analogues of (3) is illustrated in Scheme 4, wherethe hydroxyalkyl phenone is formed in a Friedel-Crafts reaction withisobuturyl chloride as described in U.S. Pat. No. 5,744,512.

The synthesis of Irgacure 2959 has previously been described elsewhere(German Offenlegungsschrift 3.512.179). The attachment ofphotoinitiators with similar structure as Irgacure 2959 onto apolyalkylether is the main focus of the present invention. Following thesynthetic route in Scheme 4, it will be possible to place specificsubstituents on the benzene ring by methods generally known in the art.

Derivatives of Irgacure 2959 are characterized as Type Iphotoinitiators, and other photoinitiators that fall in this categoryare benzoinethers, benzil ketals, α-dialkoxy-acetophenones,α-hydroxy-alkyl-phenones, α-amino-alkyl-phenones and acyl-phosphineoxides. A further example of a polymeric photoinitiator based on abenzoinether is illustrated in Scheme 5.

Depicted in Schemes 3-5 are examples of Type I photoinitiators attachedto a polyalkylether backbone and an example of the preparation of a TypeII polymeric photoinitiator is shown in Scheme 6 with xanthones,thioxhanthones and acridones as the photoinitiator moiety itself.

The preparation of the polymeric photoinitiator shown in Scheme 6,follows the same principles as shown in Schemes 3-5, where a hydroxyfunctionality present on the photoinitiator is reacted withepichlorhydrin. The resulting compound is then co-polymerized with asubstituted epoxide thus resulting in the polymeric photoinitiator.Preparation of various substituted xanthone, thioxhanthone and acridonemolecules are detailed in J. Zhao, R. C. Larock J. Org. Chem. 72 (2007),583-588. R″₁, and R″₂ may be selected from the same set of functionalgroups as R′₁, R′₂, R′₃ and R′₄.

As another example of a type II polymeric photoinitiator, a benzophenonesubstituted polyethylene oxide is illustrated in Scheme 7.

Synthesis of the epoxide derivatized benzophenone has been described forother analogues than the pure benzophenone in U.S. Pat. No. 4,376,788.No details were given for the intermediate in Scheme 7. The epoxide cansubsequently be polymerized into the polyethylene oxide substitutedbenzophenone.

An alternate route to derivatized polyethylene oxide could be viagrafting techniques as exemplified in Scheme 8.

Grafting of the peroxy ester is catalysed by copper(I) as described inJ. March: “Advanced Organic Chemistry. Reaction, Mechanisms, andStructure”, 3. ed., p. 636-7, Wiley-Interscience, New York, 1985). Thespecific example shown in Scheme 8 is also disclosed in WO2008071796.

Polyalkyl Oxide Derived Photoinitiators.

A general scheme for preparation of polyalkyl oxide derivedphotoinitiators is shown in Scheme 9, where the polymer is synthesizedthrough an acyclid diene polymerization (ADMET) reaction.

Such polymerization types have been described in K. B. Wagener, K.Brzezinska Macromolecules, 24 (1991), 5273-5277.

Considerable research has been focused on polymerizing substitutedoxiranes, with different initiators and different solvents. Thus(4-(oxiran-2-ylmethoxy)phenyl)(phenyl)methanone can most likely bepolymerized with e.g. potassium t-butoxide as an initiator in a anionicpolymerization scheme as done in P. Yang, X. Zhu, Y. Yo, Y. M. Xia andT. Li Jour. Appl. Polym. Sci. 113 (2009), 3656-3660. Reaction conditionsfor similar polymerizations with a variety of other nucleophiles such aspotassium hydroxide as initiators are presented in J. Cao, N.-F. Yang,P.-D. Wang and L.-W. Yang Polymer International, 57 (2008), 530-537.Several reaction conditions are published in patent literature as well,where in U.S. Pat. No. 4,472,560 metal cyanide complexes are used ascatalysts for the epoxide polymerization. Organoaluminium catalysts arealso described in U.S. Pat. No. 4,009,128 to work well in a cationicpolymerization scheme.

Example 1 Synthesis of (4-(oxiran-2-ylmethoxy)phenyl)(phenyl)methanone

A solution of 4-hydroxy-benzophenone (15.02 g, 75.78 mmol) was dissolvedin ethanol (75 mL) and NaOMe (4.11 g, 76.09 mmol) in methanol (75 mL)was added. The reaction mixture was stirred for 20 min. at roomtemperature and then concentrated by evaporation on a rotavapor. Theresidue was dissolved in dimethylformamide (150 mL) and epichlorohydrin(10.73 g, 116 mmol) was added. After stirring for 4 hours at 110° C. anda crude product was obtained by adding activated charcoal, filtering andremoval of the solvent. The crude product was recrystallized fromethanol leaving a white compound (12 g) in 62% yield. ¹H-NMR (CDCl₃, RT,300 MHz): 7.81 (d, 2H, J=9 Hz), 7.74 (d, 2H, J=7 Hz), 7.55 (t, 1H, J=8Hz), 7.45 (t, 2H, J=7 Hz), 6.97 (d, 2H, J=9 Hz), 4.32 (dd, 1H, J₁=11 Hz,J₂=3 Hz), 3.99 (dd, 1H, J₁=11 Hz, J₂=6 Hz), 3.37 (m, 1H), 2.91 (t, 1H,J=5 Hz), 2.76 (dd, 1H, J₁=5 Hz, J₂=3 Hz); ¹³C-NMR (CDCl₃, RT, 75 MHz):195.4, 161.9, 138.0, 132.4, 131.9, 130.5, 129.6, 128.1, 114.1, 68.8,49.8, 44.5.

Polymerization Example 1 Synthesis ofpoly-co-ethyleneoxid-(4-(oxiran-2-ylmethoxy)phenyl)(phenyl)methanone

Prior to the polymerization, (4-(oxiran-2ylmethoxy)phenyl)(phenyl)methanone was carefully dried under vacuum andthen transferred into a dry round bottom flask under nitrogen. Theinitiator for the polymerization was prepared by condensing dry THF intoa round bottom flask. Naphthalene and potassium were dissolved underargon atmosphere and the solution was stirred for one day giving a darkgreen solution. Biphenyl methane was added and the resulting solutionstirred for three days giving a deep red initiator solution.

The glassware used for the polymerization was dried thoroughly andassembled in a nitrogen glove box. When attached to the synthesis setupthe glassware was flushed with argon several times prior to use. Theethylene oxide was condensed from a pressured can into a round bottomflask and dried intensively.

(4-(oxiran-2 ylmethoxy)phenyl)(phenyl)methanone (0.8 g, 3 mmol) wasweighed into a reaction flask under nitrogen and the flask was driedover sodium-potassium alloy for at least 24 hours. THF (250 mL) wascondensed into a reaction flask followed by ethylene oxide (11.1 g,0.252 mol). Afterwards the initiator solution was added with an argonrinsed syringe. The reaction flask was set into a 60° C. thermostatedwater bath for 3-5 days. If a precipitate had formed during the reactionit was filtered from the reaction solution. The reaction solution wasconcentrated by taking off a part of the reaction solvent by reducedpressure. The polymer (1) was precipitated from cold diethyl ether anddried at 40° C. for at least 24 hours. Yield: 32 wt %. MW 34000, PD 1.5(as determined by GPC). ¹H-NMR (CDCl₃, 300 K, 500 MHz): 7.85-7.79 (m,2H), 7.78-7.73 (m, 2H), 7.61-7.53 (m, 1H), 7.51-7.43 (m, 2H), 7.02-6.95(m, 2H), 4.24-4.18 (m, 2H), 3.93-3.82 (m, 4H), 3.80-3.35 (m, 243H). Theratio of benzophenone to ethylene oxide is thus ˜1:61.

Polymerization Example 2 Synthesis ofpoly-co-ethyleneoxid-(4-(oxiran-2-ylmethoxy)phenyl)(phenyl)methanone

The bench-scale polymerizations were carried out in a 250 ml glassreactor by using tri-isobutylaluminium and potassium t-butoxide as thecatalyst system (see the used standard polymerization conditions in thefollowing table).

Parameter Value Process type Slurry Solvent Toluene Monomer Ethyleneoxide, 4-(oxiran-2- ylmethoxy)phenyl)(phenyl)methanone Monomer feed typeStepwise, by volume Temperature (° C.) 20 Total Pressure (bar) 0.3-0.8

The catalyst synthesis and polymerizations of ethylene oxide were doneby using a procedure in EP1566397.

-   -   A 250 ml glass reactor was used.    -   Work was done in every step under inert conditions under a        nitrogen atmosphere; the final polymer is also stored under        nitrogen (which has not been in contact with air/water).    -   The addition of ethylene oxide was done by using a 100 ml        sight-glass by using liquid ethylene oxide (under pressure).    -   The final polymer was washed with hexane, filtered and dried        under reduced pressure at room temperature.

A polymerization was performed using toluene (60 mL), potassiumt-butoxide (56 mg), triisobutyl aluminium (1 M in hexane, 1.4 mL),ethylene gas (26 g) and 4-(oxiran-2-ylmethoxy)phenyl)(phenyl)methanone(1.3 g). The yield of the reaction was 9.1 g of the target polymer (2).¹H-NMR (CDCl₃, 300 K, 300 MHz): 7.94-6.64 (m, 9H), 4.30-3.00 (m, 121H).The ratio of benzophenone to ethylene oxide is thus ˜1:29.

Curing Example Curing ofpoly-co-ethyleneoxid-(4-(oxiran-2-ylmethoxy)phenyl)(phenyl)methanonefrom Example 1

An oblate of pristine poly-co-ethyleneoxid-(4-(oxiran-2ylmethoxy)phenyl)(phenyl)methanone (1) was placed between the two platesin a rheometer (parallel plate configuration, bottom plate is a quartzglass plate). The distance between the plates was set to 0.3 mm and thetemperature to 120° C. The measurements were run with fixed strain of 1%and a constant frequency of 1 Hz. When the loss and storage modules hadstabilized, a UV-lamp was turned on, thus irradiating the sample throughthe bottom plate on the rheometer via a fibre from the lamp. The lossand storage modules were then followed as a function of time, while theUV-lamp irradiated the sample. An illustrative result of such ameasurement is shown in FIG. 4.

The comparatively low moduli prior to irradiation with UV light indicatea low viscosity in accordance with a fairly low molecular weight of thepolymer of ˜20 kDa. However, the loss modulus is larger than the storagemodulus showing liquid behaviour of the melt. When the UV source isturned on, the moduli increase, suggesting that cross-linking reactionsare initiated which forms a covalently-bonded network of polyethyleneoxide chains. Eventually, the storage modulus becomes larger than theloss modulus showing that a solid or gel has formed as a result of thecuring reaction. As a visual inspection, the cross-linked polyethyleneoxide oblate was placed in water which resulted in a swelled gel.

Curing Example Curing ofpoly-co-ethyleneoxid-(4-(oxiran-2-ylmethoxy)phenyl)(phenyl)methanonefrom Example 2

An oblate of pristine poly-co-ethyleneoxid-(4-(oxiran-2ylmethoxy)phenyl)(phenyl)methanone (2) was placed between the two platesin a rheometer (parallel plate configuration, bottom plate is a quartzglass plate). The distance between the plates was set to 0.3 mm and thetemperature to 120° C. The measurements were run with fixed strain of 1%and a constant frequency of 1 Hz. When the loss and storage modules hadstabilized, a UV-lamp was turned on, thus irradiating the sample throughthe bottom plate on the rheometer via a fibre from the lamp. The lossand storage modules were then followed as a function of time, while theUV-lamp irradiated the sample. An illustrative result of such ameasurement is shown in FIG. 5.

As seen from FIG. 4, the polymer indeed responds to UV irradiation,however, the initial values of the modulus indicates a solid (dampingfactor is lower than one). Upon exposure to UV irradiation the sampleshows an increase in both storage and loss modulus indicating thatcross-linking is taking place.

The invention claimed is:
 1. A polyalkylether photoinitiator of thegeneral formula IR₁(A₁)_(r)-(R₂((—CH₂CH₂O)_(t)A₂)_(m)-O)_(o)—(R₃((CH₂CH₂O)_(t)A₃)_(n)-O)_(p)—R₄(A₄)_(s)  Iwherein R₂ and R₃ are independently at each occurrence identical ordifferent, linear or branched alkylene or cycloalkylene groups; whereinR₂ and R₃ may be substituted with one or more substituents selected fromCN; azides; esters; ethers; amides; halogen atoms; sulfones; sulfonicderivatives; NH₂ or Nalk₂, where alk is any C₁-C₈ straight chain alkylgroup, C₃-C₈ branched or cyclic alkyl group; R₁ and R₄ are independentlyat each occurrence identical or different, linear or branched alkyl orcycloalkyl groups or aryl groups or are independently at each occurrenceselected from H, OH, CN, halogens, amines, amides, alcohols, sulfonesand derivatives thereof, sulfonic acid and derivatives thereof,sulfoxides and derivatives thereof, carbonates, isocyanates, nitrates,acrylates, polyethylenes, polypropylenes, polyesters, polyamides,polyacrylates and polyurethanes; and when R₁ and R₄ are alkyl and arylgroups, each of R₁ or R₄ may be substituted with one or moresubstituents selected from CN; OH; azides; esters; amides; halogenatoms; sulfones; sulfonic derivatives; NH₂ or Nalk₂, where alk is anyC₁-C₈ straight chain alkyl group, C₃-C₈ branched or cyclic alkyl group;o and p are each a real number from 0-5000 provided that o+p>0; m and nare each a real number from 0-10, provided that m+n>0; r and s are eacha real number from 1-5; t is an integer from 1-100; and A₁, A₂, A₃ andA₄ are identical or different photoinitiator moieties.
 2. Thepolyalkylether photoinitiator according to claim 1, wherein A₁, A₂, A₃and A₄ are linked to R₁, R₂, R₃, and R₄, respectively, via a spacergroup.
 3. The polyalkylether photoinitiator according to claim 2,wherein the spacer group is selected from the group consisting ofalkylene, cycloalkylene, aryl, and alkylene ether groups.
 4. Thepolyalkylether photoinitiator according to claim 1, wherein R₂=—CH₂CH₂,in which one or more H atoms may be replaced by A₂.
 5. Thepolyalkylether photoinitiator according to claim 1, wherein R₃=—CH₂CH₂,in which one or more H atoms may be replaced by A₃.
 6. Thepolyalkylether photoinitiator according to claim 1, whereinR₂=—CH(CH₃)CH₂—, in which one or more H atoms may be replaced by A₂. 7.The polyalkylether photoinitiator according to claim 1, whereinR₃=—CH(CH₃)CH₂—, in which one or more H atoms may be replaced by A₃. 8.The polyalkylether photoinitiator according to claim 1, wherein R₁=OH.9. The polyalkylether photoinitiator according to claim 1, wherein R₄=H.10. The polyalkylether photoinitiator according to claim 1, wherein A₁,A₂, A₃ and A₄ are identical or different photoinitiator moietiesselected from the group consisting of benzoin ethers, phenylhydroxyalkyl ketones, phenyl aminoalkyl ketones, benzophenones,thioxanthones, xanthones, acridones, anthraquinones, fluorenones,dibenzosuberones, benzils, benzil ketals, α-dialkoxy-acetophenones,α-hydroxy-alkyl-phenones, α-amino-alkyl-phenones, acyl-phosphine oxides,phenyl ketocoumarins, silane, maleimides, and derivatives thereof. 11.The polyalkylether photoinitiator according to claim 10, wherein A₁, A₂,A₃ and A₄ are identical or different photoinitiator moieties selectedfrom the group consisting of 2-hydroxy-2-methyl-propiophenone,benzophenone, thioxanthone, benzil, anthraquionone, camphorquinone,benzoin ether, acylphosphine oxide, silane, and derivatives thereof. 12.The polyalkylether photoinitiator according to claim 11, wherein A₁, A₂,A₃ and A₄ are identical photoinitiator moieties.
 13. The polyalkyletherphotoinitiator according to claim 1, wherein A₁, A₂, A₃ and A₄ are atleast two different photoinitiator moieties.
 14. The polyalkyletherphotoinitiator according to claim 12, wherein at least one of A₁, A₂, A₃and A₄ is a benzophenone photoinitiator moiety.
 15. The polyalkyletherphotoinitiator according to claim 14, wherein at least A₂ and A₃ arebenzophenone photoinitiator moieties.
 16. The polyalkyletherphotoinitiator according to claim 1, wherein o and p are each from0-3000.
 17. The polyalkylether photoinitiator according to claim 1,wherein m and n are each an integer from 0-8 provided that m+n>0. 18.The polyalkylether photoinitiator according to claim 1, wherein m=1and/or n=1.
 19. The polyalkylether photoinitiator according to claim 1,wherein m=1, n=0 and the ratio o:p is at least 1:1000.
 20. Thepolyalkylether photoinitiator according to claim 1, wherein r and s areeach from 1-4.
 21. The polyalkylether photoinitiator according to claim1, having a molecular weight between 5 kDa and 10,000 kDa.
 22. A methodfor curing a matrix composition, said method comprising the steps of: a)mixing the polyalkylether photoinitiator according to claim 1 with amatrix composition; b) curing the mixture obtained in step a) byexposing it to UV radiation.
 23. A matrix composition obtained via themethod of claim 22.